The heart includes four natural valves that function to regulate flow direction as blood is pumped between the lungs and various blood vessels comprising the circulatory system of the body. Pumping of the blood is primarily carried out by contraction and expansion of the ventricles, the lowermost heart chambers.
The mitral and tricuspid valves interconnect the left and right atria, respectively, and their corresponding ventricles. Flow is from the atrium to the ventricle. When the ventricle contracts, the resulting pressure differential closes the valve between the chambers so that blood is forced outwardly from the heart. When the ventricle relaxes, the reduction in intraventricular pressure to a lesser level than the atrial pressure causes the valve to open and blood to flow from the atrium into the ventricle. The mitral and tricuspid valves, which are known as the atrioventricular or intraflow valves, thus operate to prevent backflow into the atria during ventricular contraction while permitting blood to flow therethrough during ventricular relaxation.
The aortic and pulmonary valves, on the other hand, are known as semilunar or outflow valves and are located where blood leaves the heart. The aortic valve interconnects the left ventricle with the aorta, while the pulmonary valve is located at the junction between the right ventricle and pulmonary artery.
A relatively common human ailment is failure of one or more valves of the heart. In many cases, the diseased or defective valve can be surgically removed and replaced with a prosthetic valve, thereby improving the life of the patient. Although serious, such valve replacement surgery has become commonplace in recent years.
Various prosthetic heart valves have been developed for this purpose. Such valves can be grouped into three general types with each prosthetic valve including either a ball, disc or leaflet type flow regulating device supported by a valve housing that is fixed in place by means of suturing between the natural valve annulus and the prosthetic valve sewing ring. For example, U.S. Pat. No. 3,589,392 to Meyer shows a valve of the split leaflet type wherein the separable leaflet portions are generally arcuate in cross section and are tethered or hinged at their bases to the valve body in a symmetric manner by means of an insertable ring member. U.S. Pat. No. 4,254,508 to Bokros discloses a bileaflet heart valve wherein flat leaflets are pivotally supported by means of pin and slot connections. The heart valve prosthesis of U.S. Pat. No. Re. 30,507 to Kaster and that of U.S. Pat. No. 4,021,863 to Woien utilize free-floating pivoting circular discs that can also rotate, while the valve in U.S. Pat. No. 4,306,319 to Kaster incorporates a pivoting non-circular disc that cannot rotate. U.S. Pat. No. 3,113,586 to Edmark shows a valve of the flexible leaflet type. U.S. Pat. Nos. 4,030,142 to Wolfe and 3,513,485 to Davila relate to prosthetic heart valves utilizing movable occluder elements that are centrally supported within a housing ring or base.
Of course a prosthetic heart valve must be compatible with and acceptable by the heart and blood; however, various difficulties can arise due to deficiencies in design, operational characteristics, physical structure, and structural materials. Some of the prior designs have been functionally inefficient, unresponsive and resistive of the free passage of blood. Others cause blood stagnation and turbulence that results in the formation of blood clots on the valve structure that impede the normal valving movements of the prosthesis. Elevated levels of hemolysis (destruction of red blood cells) can occur in some valves that have rough surfaces and/or small or narrow flow passages. Hemolysis also can occur in valves having hard and substantial contact surface areas or interfaces such as between a stationary valve member and a mobile valve member or occluder. In such valves, blood cells become trapped between these surfaces and are mechanically crushed thus reducing their useful life span.
The operational characteristics of some early prosthetic valves cause elevated transvalvular pressure gradients, diminished stroke volume and record small functional or effective orifice areas. Blood flow through the natural healthy valve(s) is directed centrally downstream; however, blood flow through a caged-ball or caged-disc valve is directed radially from the center of the valve by the respective occluder. The subsequently developed pivoting disc valves permit the blood to flow nearly centrally downstream but deflected from the central axis an amount equal to the design angle of the disc occluder in the open position. Improvement in valvular hemodynamic efficiency continues to be made but no prosthetic valve functions as efficiently as the normal healthy cardiac valve(s).
Although the earlier developed caged-ball concept is generally recognized as a predictable prosthesis, it is also known to be an excessively bulky valve due to its relatively large physical cage structure. In some patients implanted with a caged-ball valve where there is limited space for such bulky valve structures, the extending cage can engage the internal wall surface of heart and disrupt the natural beating and pumping rhythm which in turn can frequently cause cardiac arrest. The lower profile disc valves represent a reduction in physical structure and bulk and thus do not involve some of the problems associated with cage-ball valves.
With respect to materials, except for the supporting struts of some tissue valves and the sewing ring of most all valves, plastics have not been found to perform satisfactorily. Silicone rubber when used in the construction of the ball of some caged-ball valves or the disc of some caged-disc valves have been known to deteriorate due to wear and chemical degradation. Other plastic materials, such as TEFLON, DELRIN, LUCITE, polypropylene and others have recorded equally disappointing results in prosthetic heart valves of various designs.
A need thus exists for a new prosthetic heart valve of improved design, performance and construction over the prior art.